1. Field of the Invention
The present invention relates generally to polymeric materials, and more specifically to halogenated polymeric materials useful in the construction of devices for telecommunications.
2. Technical Background
In optical communication systems, messages are transmitted by electromagnetic carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems.
One preferred device for routing or guiding waves of optical frequencies from one point to another is an optical waveguide. The operation of an optical waveguide is based on the fact that when a light-transmissive medium is surrounded or otherwise bounded by an outer medium having a lower refractive index, light introduced along the axis of the inner medium substantially parallel to the boundary with the outer medium is highly reflected at the boundary, trapping the light in the light transmissive medium and thus producing a guiding effect between channels. A wide variety of optical devices can be made which incorporate such light guiding structures as the light transmissive elements. Illustrative of such devices are planar optical slab waveguides, channel optical waveguides, rib waveguides, optical couplers, optical splitters, optical switches, optical filters, arrayed waveguide gratings, waveguide Bragg gratings, variable attenuators and the like. For light of a particular frequency, optical waveguides may support a single optical mode or multiple modes, depending on the dimensions of the inner light guiding region and the difference in refractive index between the inner medium and the surrounding outer medium.
Optical waveguide devices and other optical interconnect devices may be constructed from organic polymeric materials. Whereas single mode optical devices built from planar waveguides made from glass are relatively unaffected by temperature, devices made from organic polymers may show a significant variation of properties with temperature. This is due to the fact that organic polymeric materials have a relatively high thermo-optic coefficient (dn/dT). Thus, as an organic polymer undergoes a change in temperature, its refractive index changes appreciably. This property can be exploited to make active, thermally tunable or controllable devices incorporating light transmissive elements made from organic polymers. One example of a thermally tunable device is a 1xc3x972 switching element activated by the thermo-optic effect. Thus, light from an input waveguide may be switched between two output waveguides by the application of a thermal gradient induced by a resistive heater. Typically, the heating/cooling processes occur over the span of one to several milliseconds.
Most polymeric materials, however, contain carbon-hydrogen bonds, which absorb strongly in the 1550 nm wavelength range that is commonly used in telecommunications applications, causing devices made from such materials to have unacceptably high insertion losses. By lowering the concentration of Cxe2x80x94H bonds in a material by replacement of Cxe2x80x94H bonds with Cxe2x80x94D or C-halogen bonds, it is possible to lower the absorption loss at infrared wavelengths. While planar waveguides made from fluorinated polyimides and deuterated or fluorinated polymethacrylates have achieved single mode losses of as little as 0.10 dB/cm at 1300 nm, it is relatively difficult to make optical devices from these materials. For example, the processes by which these waveguides have typically been made includes the use of a reactive ion etching process, which is cumbersome and can cause high waveguide loss due to scattering. Further, deuteration is not an effective means of reducing loss in the 1550 nm wavelength range. Fluorinated polyimides and deuterated or fluorinated polymethacrylates have higher losses in the telecommunications window near 1550 nm, typically on the order of 0.6 dB/cm. Oxe2x80x94H and Nxe2x80x94H bonds also contribute strongly to loss at wavelengths near 1310 nm and 1550 nm. Compositions are sought in which the concentrations of Oxe2x80x94H and Nxe2x80x94H bonds are minimal.
Photopolymers have been of particular interest for optical interconnect applications because they can be patterned using standard photolithographic techniques. Photolithography involves the selective polymerization of a layer of the photopolymer by exposure of the material to a pattern of actinic radiation. Material that is exposed to the actinic radiation is polymerized, whereas material that is not exposed remains unpolymerized. The patterned layer is developed, for example, by removal of the unexposed, unpolymerized material by an appropriate solvent.
Among the many known photopolymers, acrylate materials have been widely studied as waveguide materials because of their optical clarity and low birefringence, and the ready availability of a wide range of monomers. However, the performance of optical devices made from many acrylate materials has been poor, due to high optical losses, poor resistance to aging and yellowing and thermal instability of the polymerized material. Further, acrylate materials have relatively high losses, up to 1 dB/cm, in the L-band (about 1565 nm to about 1620 nm), and do not cure efficiently in air, necessitating the use of oxygen-free conditions.
In the attachment of optical fibers to highly fluorinated, low refractive index polymer optical waveguides, a process known as pigtailing, highly fluorinated, low refractive index adhesives are desired. Most suitably, in order to reduce back reflections, the refractive index of the adhesive lies between that of the fluorinated polymer waveguide (about 1.33 to about 1.40) and the optical fiber (about 1.46). Materials that are known to UV cure in the presence of air, such as epoxies and vinyl ethers, generally have higher refractive indices (about 1.47 to about 1.52). Further, these materials are difficult to formulate with highly fluorinated monomers to reduce their refractive index due to the insolubility of the required cationic photoinitiators in nonpolar materials.
One aspect of the present invention relates to an energy curable composition including an at least difunctional thiol compound, an at least difunctional ethylenically unsaturated compound having a perfluorinated moiety, and a selected amount of a free radical initiator wherein at least one of the thiol compound and the ethylenically unsaturated compound is at least partially halogenated, the ratio of thiol moieties to ethylenically unsaturated moieties is between about 1:2 and about 2:1, and the thiol compound and the ethylenically unsaturated compound account for between about 35% and about 99.9% of the energy curable composition.
Another aspect of the present invention relates to a polymeric material having thioether moieties in a concentration of at least about 0.05 M; and at least one perhalogenated moiety.
Another aspect of the present invention relates to an optical element having a polymeric core, said core including a polymeric material including thioether moieties in a concentration of at least 0.05 M and at least one at least partially halogenated moiety.
Another aspect of the invention relates to a method of making an optical element including the steps of (a) applying a layer of a cladding composition to a substrate, the clad composition including an at least difunctional thiol compound, an at least difunctional ethylenically unsaturated compound, and an effective amount of a free radical initiator wherein at least one of the thiol compound and the ethylenically unsaturated compound is at least partially halogenated, the ratio of thiol moieties to isolated ethylenically unsaturated moieties is between about 1:2 and about 2:1 and the ethylenically unsaturated compound account for between about 35% and about 99.9% of the cladding composition; (b) at least partially curing the cladding composition to form a polymeric cladding layer; (c) applying a photosensitive core composition to the surface of the polymeric cladding layer to form a core composition layer, the core composition including an at least difunctional thiol compound, an at least difunctional ethylenically unsaturated compound, and an effective amount of a free radical initiator wherein at least one of the thiol compound and the ethylenically unsaturated compound is at least partially halogenated, the ratio of thiol moieties to isolated ethylenically unsaturated moieties is between about 1:2 and about 2:1 and the ethylenically unsaturated compound account for between about 35% and about 99.9% of the core composition; (d) imagewise exposing the photosensitive core composition layer to sufficient actinic radiation to effect the at least partial polymerization of an imaged portion and to form at least one non-imaged portion of the photosensitive core composition layer; (e) removing the at least one non-imaged portion without removing the imaged portion, thereby forming a polymeric patterned core from the imaged portion; (f) applying a photosensitive overclad composition onto the polymeric patterned core; and (g) at least partially curing the overclad composition to form a polymeric overclad layer, wherein the polymeric overclad layer and the polymeric cladding layer have a lower refractive index than the polymeric patterned core.
The compositions, methods, and devices of the present invention result in a number of advantages over prior art compositions, methods, and devices. For example, the compositions of the present invention are far less sensitive to oxygen during cure than analogous acrylate materials, and can be formulated to cure substantially completely in the presence of air, thus obviating the need for excluding oxygen during processing, such as in the photolithographic exposure step of the waveguide manufacturing process. The polymerized compositions of the present invention have a low optical loss throughout both the C-band and the L-band, allowing for the construction of optical waveguide devices suitable for use in either band. Since they cure in air, these compositions may also be used as adhesives, for example, in the pigtailing of optical fibers to waveguide devices. The compositions of the present invention may be formulated to have a low refractive index, making them especially useful in the pigtailing of optical fibers to highly fluorinated polymer optical waveguide devices.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention.